The present invention relates to a technology for producing zeolite from incineration ash or a composition containing aluminosilicate as a raw material.
Conventionally, fly ash generated in the course of coal burning and a composition containing aluminosilicate have been used as raw materials for producing artificial zeolite. Japanese Unexamined Patent Publication No. 6-321525 and No. 6-321526 disclose methods and apparatuses for producing zeolite from such raw materials by means of a hot aqueous alkaline solution.
Japanese Unexamined Patent Publication No. 10-324518 discloses a method of continuously producing artificial zeolite by a circulating fluidized-bed as well as a device therefor.
In the conventional methods of producing artificial zeolite, a mixture formed by mixing fly ash or a composition containing aluminosilicate with an alkali is heated by radiation heat or conduction heat. The thermal energy density has limits to increase, which often makes a heating time longer. Further, the reaction speed of forming zeolite with a hot aqueous alkaline solution is slow, thereby taking a longer time to produce artificial zeolite. In other words, heating is performed from outside of particles, and therefore a longer time is needed for heat influx into the particles and for diffusion of an alkali, resulting in a longer reaction time as a whole.
In addition, the zeolite generated by the thermal alkaline reaction forms crust over the particle surface, retarding the reaction inside the particles. Thus, the formation of zeolite is hindered, and the zeolite conversion rate is not high.
Furthermore, in the conventional methods, a large amount of sodium hydroxide which is not involved in the reaction requires a large amount of work and time in recovering and reutilizing the alkali, making it difficult to produce artificial zeolite having a high function at a low cost.
There is a further problem from a viewpoint of energy saving since the conventional methods require the steps of separating artificial zeolite from an alkaline solution, and washing and drying the zeolite, and the separating and drying steps need a large amount of energy, resulting in a high production cost.
An object of the present invention is to provide a technology which enables production of artificial zeolite by a simpler process, compared with the conventional technology, with a reduced amount of alkali used and discharged at a lower energy expenditure, and a reduced time for production.
In order to solve the above-described problems, the method for producing zeolite according to the present invention comprises adding an aqueous alkaline solution to incineration ash or a composition containing aluminosilicate to prepare a mixture in the form of slurry or mud, heating the mixture, and directly irradiating the mixture with electromagnetic waves having frequencies ranging from 300 MHz to 30 GHz, while continuously moving the mixture, to form the zeolite. By employing this process, an alkali in a minimum amount necessary for the reaction penetrates into the particle solid phase of incineration ash or the like, and the alkali instantly forms zeolite by means of the heat generated from inside the particles by the electromagnetic wave irradiation. This process, therefore, enables the production of artificial zeolite in a short time, with a reduced amount of alkali used and discharged. Further, the conventional solid-liquid separation/purification processes are not necessary, thereby simplifying the whole process.
It is noted that incineration ash or a composition containing aluminosilicate may include natural zeolite and artificial zeolite produced by other production methods which have not been converted into phillipsite. Accordingly, the present invention can be applied to improving the properties of natural or other zeolite.
Since the electromagnetic wave irradiation causes heat generation only in the mixture of incineration ash or the like, virtually without heating the surrounding devices, the atmospheric gases, etc., the heat efficiency is high and the energy expenditure can be lowered. Furthermore, the electromagnetic wave is irradiated after the mixture is heated, thereby raising the heat conversion efficiency to approximately 70%. As this preliminary heating process, the mixture is preferably heated at a temperature ranging from 80 to 150xc2x0 C.
The principal component of the zeolite produced according to the present invention is phillipsite. It may also include faujasite, zeolite A, hydroxy sodalite, etc. with non-zeolite components, that is, components other than zeolites, such as unburned carbon, iron, etc.
It is noted that the incineration ash described in the present invention is incineration ash made of compositions containing aluminosilicate. The incineration ash includes coal ash, incineration ash of sludge generated in paper manufacturing, incineration ash from activated sludge produced by sewage disposal, incineration ash from city garbage, incineration ash of solidified fuel made from garbage or the like. The composition containing aluminosilicate refers to a mineral containing a salt formed by partially substituting silicate or silicon dioxide with aluminum. Such mineral is exemplified as orthoclase, anorthite, analcime, chabazite and mica.
By irradiating the electromagnetic waves having frequencies ranging from 300 MHz to 30 GHz, the dipole moments of water molecules existing in the mixture of incineration ash or a composition containing aluminosilicate and an aqueous alkaline solution vibrate vigorously (from several hundred million to several billion times per second) to give a high temperature by generating heat inside the particles of incineration ash or the like, which promotes a hot alkaline reaction instantly. Thus, the reaction to form zeolite, which took several hours to several tens of hours according to the conventional methods, can be completed in several minutes.
Since the mixture of incineration ash or a composition containing aluminosilicate and an aqueous alkaline solution is in the form of slurry or mud, the handling and transportation efficiency in the production process are improved. Further, an efficient heat generation can be realized by the electromagnetic wave irradiation. Thus, the amount of alkali can be adjusted to a minimum necessary for the reaction, resulting in great reduction of the amount of discharged waste alkali.
The reaction to form zeolite nuclei which determines the rate of overall reaction to form zeolite is accelerated by adding particles for nucleus formation in advance to incineration ash or a composition containing aluminosilicate. Thus, the reaction speed of forming, zeolite is raised by three to five times, thereby producing artificial zeolite in a shorter time. As the particles for nucleus formation, zeolite particles, glass powder, etc. are suitable.
Since a mixture of incineration ash or a composition containing aluminosilicate and an aqueous alkaline solution is continuously moved during irradiation of electromagnetic waves, the control and adjustment of the irradiation conditions are easier, allowing continuous working of the overall process for producing zeolite. Accordingly, the efficiency of the process is enhanced. It is noted that, if the mixture of incineration ash or the like has edges or protrusions, the electric field tends to concentrate on the edges or protrusions, causing uneven heating. It is preferable to employ a cylindrical rotary heater or a conveyor-type heater which can move the heating surface up and down to prevent such uneven heating.
By using, as incineration ash, fly ash formed in the course of coal burning or garbage incineration, industrial wastes can be converted into useful resources. Fly ash formed in the course of coal burning refers to minute ash particles collected by a dust collector in the course of coal burning using a pulverized coal-fired boiler. The ash particles include silicon oxide, aluminum oxide, calcium oxide, etc., with an ignition loss of 5% or less and a specific gravity of 1.9 or more, having a particle size distribution passing 75% or more of the particles through a 44 xcexcm standard sieve. Fly ash generated by garbage incineration has a composition containing silica, alumina and lime, which is similar to the fly ash generated by coal burning.
The apparatus for producing zeolite according to the present invention is an apparatus comprising: kneading means for adding an aqueous alkaline solution to incineration ash or a composition containing aluminosilicate to form a mixture and for kneading the mixture; heating means for heating the mixture kneaded by the kneading means; electromagnetic wave irradiation means for irradiating the heated mixture with electromagnetic waves to form zeolite; cleaning means for cleaning the zeolite formed by the electromagnetic wave irradiation; and drying means for drying the cleaned zeolite.
The apparatus having the above construction realizes a simpler process for producing artificial zeolite, compared with the conventional technology, with a reduced amount of alkali used and discharged, at a lower energy expenditure, and a reduced time for production.